Field of the Invention: This invention relates to a method for the manufacture of a continuous sheet of glass by supporting molten glass on a pool of molten metal while forming and cooling the glass. More particularly, this invention relates to a method for cooling the molten metal in a manner so as to enhance the lifting of the sheet of glass upwardly from the pool of molten metal and to produce glass of superior flatness.
Description of the prior art: Molten glass may be delivered onto molten metal and formed into a continuous sheet or ribbon of glass according to the teachings of Heal, U.S. Pat. No. 710,357 or of Hitchcock, U.S. Pat. No. 789,911, according to the teachings of Pilkington, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816 or according to the teachings of Edge and Kunkle, application Ser. No. 338,497, filed Mar. 6, 1973. These patents describe processes in which continuous sheets of flat glass are formed while the glass being formed is supported on molten metal. After a continuous sheet of glass has been formed, it has been shown to be removed from the surface of a pool of supporting molten metal by lifting it slightly and conveying it away from the pool of molten metal. U.S. Pat. No. 3,233,995 to Javaux and U.S. Pat. No. 3,420,647 to Charnock describe the removal of a partially formed layer of glass from the surface of the pool of molten metal and drawing and forming this into a continuous sheet of glass by drawing it upwardly from the surface of the pool of molten metal in the manner of the Pittsburgh, Colburn or Fourcault processes.
In a process wherein a finished continuous sheet of glass is lifted from the surface of a pool of molten metal on which it is formed there is invariably some affinity for the molten metal to the formed sheet of glass. This is most pronounced when the molten metal, for example, molten tin, becomes partially oxidized and adheres to the newly formed glass as dross. There is also an affinity between such molten metals in a reduced state and a freshly formed surface of glass. This is evident in the fact that flat glass produced by supporting the glass on molten metal using any of the known processes for doing so has approximately an order of magnitude higher tin concentration in its surface region that has been adjacent to molten metal during forming as in its other surface region that has been facing away from the molten metal during forming.
It has been observed that the bottom surface of glass which has been in contact with molten metal during its formation is characterized by a series of very fine transverse ridges and valleys extending across the width of the sheet and repeating periodically along its length in a direction of glass movement along the surface of the molten metal and away from the pool of molten metal. It has now been discovered that these transverse ridges and valleys can be substantially eliminated from the surface of flat glass that has been in contact with molten metal during forming. This is accomplished most conveniently when the continuous sheet of flat glass that is formed is lifted from the surface of the pool of supporting metal at a relatively large angle, such as, for example, when the formed glass is lifted upwardly from the pool of molten metal and conveyed vertically upward therefrom.